There are many processes and environments in which it is desirable to know the ambient pressure and in which a sensor is used in order to monitor same. One such common process is during exploration and production of hydrocarbons such as oil in which it is necessary to measure the pressure of the hydrocarbons in a reservoir. Another application is the measurement of the fluid pressure associated with pumps or natural drivers for transporting such hydrocarbons from one location to another. Pressure drops across a venturi is one means by which flow of a fluid can be detected, which therefore requires detection of the pressure difference on both sides of such a venturi.
Pressures of such fluids are traditionally measured with a quartz crystal based pressure measuring devices such as that manufactured by Quartzdyne, Inc. of Salt Lake City, Utah as the Quartzdyne.TM. Series QS High Pressure Laboratory Transducer. Such a pressure measuring device measures the change in mechanical oscillation frequency associated with the elastic deformation of the crystal in response to applied pressure. Quartz is the medium of choice for such applications due to inherent long term stability, as well as its minimal creep and hysteresis properties. The change in frequency with temperature is also very predictable.
Traditionally the change in frequency of the quartz crystal is measured and compared to a reference crystal which is temperature compensated with the resulting data correlated and calibrated to a direct pressure measurement. Although the reliability of such a quartz crystal is extremely high, the electronics required to measure frequency change are subject to failure particularly when the transducer and its associated electronics are subjected to elevated temperatures such as above 125.degree. C.
Certain techniques exist for measuring pressure using a Bragg grating. However, such techniques are either complex, costly, or do not constrain the optical fiber from buckling in the grating region.
For example, a fiber optic grating based sensor is described in U.S. patent application Ser. No. 08/925,598 entitled "High Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environments" to Robert J. Maron. In that case, an optical fiber is attached to a compressible bellows at one location along the fiber and to a rigid structure at a second location along the fiber with a Bragg grating embedded within the fiber between these two fiber attachment locations and with the grating being in tension. As the bellows is compressed due to an external pressure change, the tension on the fiber grating is reduced, which changes the wavelength of light reflected by the grating. Such a sensor requires a complex bellows structure and does not constrain the fiber from buckling in the grating region.
Another example is described in Xu, M. G., et al, "Fibre grating pressure sensor with enhanced sensitivity using a glass-bubble housing", Electronics Letters, 1996, Vol. 32, pp. 128-129, where an optical fiber is secured by UV cured cement to a glass bubble at two ends with a grating inside the bubble. However, such a sensor does not constrain the optical fiber against buckling in the region of the grating.
It is also known that a grating-based pressure sensor may be made by placing a polarization maintaining (PM) optical fiber in a capillary tube having rods therein, and measuring changes in grating birefringence caused by changes in the transverse strain on the fiber grating due to transverse pressure forces acting on the capillary tube, as is discussed in U.S. Pat. No. 5,841,131, to Schroeder et al., issued Nov. 24, 1998. However, such a technique may be expensive or complex to implement.
It is therefore desirable to have a fiber optic Bragg grating pressure sensor that can measure the elastic deformation of a pressure detecting device while minimizing non-axial (or transverse) movement of the optical fiber in the region of the Bragg grating.